The chemical conversion coating of magnesium, its alloys and beryllium and its alloys is known in the art as a process whereby the surface of the metal is chemically converted to a surface that more easily accepts applied coatings, i.e. paint, and/or increases the corrosion resistance of the metal. An industry accepted process for the conversion coating of magnesium, its alloys and beryllium and its alloys involves the use of known carcinogens such as chromates (see: J. A. Hawk, Jr., R. E. Strock, Jr. and R. C. Fullerton-Batten, “Encyclopedia of Chemical Processing Design”, volume 4, CRC Press, 1977, page 324. and ASTM D1732). As a result, chromates and related toxic materials are being displaced by less toxic alternatives.
Alternative conversion coatings in current use are based upon phosphates and/or anodizing or oxidation of the metal. Oxidized or phosphate generated conversion coatings tend to be brittle. In addition the processes require the use of long involved cleaning procedures involving toxic fluorides (see: Biestek, T., and Weber, J.: “Electrolytic and Chemical Conversion Coatings”, Portculis Press. Ltd., Queensway House, 2 Queensway, Redhill Surrey, RHI 1QS, 1976 and http://www.pfonline.com/articles/019601.html).
A typical process for magnesium and its alloys would involve: Hot alkaline soak cleaner at about 180-200° F. for three to five minutes, D.I water rinse, dip in a mild acid solution to remove unwanted metal oxides, rinse in D.I. water, conversion coating in a phosphate based solution containing toxic fluorides and a final rinse (see: “Operations in Magnesium Finishing”, Dow Chemical U.S.A., Magnesium Metals Technical Service and Development, Midland, Mich. 48640). Such a surface needs to be painted immediately after being produced as it will quickly oxidize and it provides no secondary corrosion resistance.
Electroplating or non-electrolytic plating of magnesium and/or Beryllium requires all of the above steps (with the exception of the phosphate treatment) and the application of an immersion zinc coating and/or the application of a film of copper using an alkaline copper cyanide plating bath. To provide for a unifoim and even plating surface. These processes are rather difficult to properly apply and even with the use of the above processing steps, the plating of magnesium and/or Beryllium is a very difficult process and needs to be sealed to prevent secondary corrosion problems or disbondment of the plating (see: Sakata,” Electroless Nickel Plating Directly on Magnesium Alloy Die Castings”, 74th AESF Technical Conference (1987)15).